Message-ID: <44D8B12C.40200@sw.ru>
Date: Tue, 08 Aug 2006 19:43:40 +0400
From: Kirill Korotaev <dev@sw.ru>
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To: Dave Hansen <haveblue@us.ibm.com>
CC: Andrew Morton <akpm@osdl.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] sys_getppid oopses on debug kernel
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> Accessing freed memory is a bug, always, not just *only* when slab
> debugging is on, right?  Doesn't this mean we could get junk, or that
> the reader could potentially run off a bad pointer?
no, read the comment in sys_getppid.
It is a valid optimization. _safe_ and alowing to bypass taking the lock.
BUT! This optimization relies on the fact that kernel memory (DMA + normal zone)
is always mapped into virtual address space.
Which is invalid for debug kernels only.

> It seems that this patch only papers over the problem in the case when
> it is observed, but doesn't really even fix the normal case.
> 
> Could we use a seqlock to determine when real_parent is in flux, and
> re-read real_parent until we get a consistent one?  We could use in in
> lieu of the existing for() loop.

Kirill

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